1. Field of the Invention
The present invention relates to a voltage reduction detection circuit that detects a voltage drop (voltage reduction) in an AC (alternating-current) power supply that feeds AC power to a switching power supply. The present invention also relates to a switching power supply system that is provided with such a voltage reduction detection circuit.
2. Description of Related Art
Conventionally, in electronic devices such as television receivers and information recording/playback apparatuses, there are employed compact, high-efficiency switching power supplies, and there are known switching power supplies with various configurations (for example, Patent Documents 1 to 4 listed below). FIG. 5 is a block diagram illustrating an example of a conventional configuration of a switching power supply of the RCC (ringing choke converter) type. As shown in FIG. 5, the conventional switching power supply 100 is provided with a rectifier circuit 101, a switching transformer 102, a switching circuit 103, a secondary rectifier circuit 104, a feedback circuit 105, a photocoupler 106, and a protection circuit 107.
The rectifier circuit 101 converts AC power fed from an AC power supply into DC (direct-current) power. The switching transformer 102 has a primary coil 102a to which a DC voltage obtained from the rectifier circuit 101 is applied and across which a DC current passes as a result, a secondary coil 102b that outputs a voltage corresponding to the voltage applied to the primary coil, and a drive coil 102c that generates a voltage corresponding to the voltage supplied to the primary side of the switching transformer 102. The switching circuit 103 turns on and off the DC voltage applied to the primary side of the switching transformer 102 to output an AC voltage from the secondary side of the switching transformer 102.
The secondary rectifier circuit 104 converts three different AC voltages (high-voltage, medium-voltage, low-voltage, in order from the top) outputted from the secondary side of the switching transformer 102 into DC voltages, respectively. The feedback circuit 105 gives the primary side a feedback based on the magnitudes of the voltages outputted from the secondary rectifier circuit 104 to stabilize the output voltages. The photocoupler 106 has a photodiode 106a and a phototransistor 106b, and supplies the feedback signal from the feedback circuit 105 to the switching circuit 103. The protection circuit 107, when an abnormality such as short circuiting occurs at the secondary side, forcibly stops the switching circuit 103 to stop the DC voltage supplied to the primary side of the switching transformer 102.
When the switching power supply 100 starts to operate, first, AC power fed from an AC power supply (generally a commercial power supply) is converted into DC power by the rectifier circuit 101, and is then fed to the switching transformer 102 via the switching circuit 103. Here, the switching circuit 103 turns the DC voltage on and off that is fed to the switching transformer 102 at a predetermined period, so that the voltage is intermittently supplied to the primary side of the switching transformer 102. At the secondary side of the switching transformer 102, three different AC voltages are generated, and the AC voltages outputted are converted into DC voltages, respectively, by the secondary rectifier circuit 104, so that three different DC voltages are outputted.
The feedback circuit 105 increases and decreases the current that passes through the photodiode 106a in accordance with to the magnitudes of the two outputted voltages, medium-voltage and low-voltage, supplied from the secondary rectifier circuit 104. For example, when a voltage higher than the rated voltage value is outputted at the secondary side, a larger current passes through the photodiode 106a, intensifying the light emission of the photodiode 106a. The intense light is then received at the base of the phototransistor 106b, causing the phototransistor to pass a larger current. Thus, when the current generated from the drive coil 102c flows into the switching circuit 103, a larger current flows into the switching circuit 103. The switching circuit 103 then lengthen the period of stopping the current supplied to the primary side of the switching transformer 102. With the above operation, the voltage generated from the secondary side of the switching transformer 102 is reduced to be closer to the rated voltage value. Note that when a voltage lower than the rated voltage value is outputted at the secondary side, the opposite operation is performed.
The protection circuit 107 forcibly stops the switching circuit 103 when an abnormality such as short circuiting occurs at the secondary side as described above. The protection circuit 107 also has a latch function so that when a stop as described above (also referred to as a “latch stop”) occurs, the switching function is kept stopped until the power supply at the primary side of the switching transformer is turned on again. In other words, when a latch stop occurs, unless the AC cable is disconnected once and a recovery is made from the abnormality, normal oscillation is not resumed.
The latch stop is provided originally as a function for coping with problems such as short circuiting occurring at the secondary side. However, in the conventional switching power supply 100, the latch stop may result even when a voltage drop occurs in a commercial power supply, or when momentary power failure (the voltage value of zero) occurs. Momentary power failure occurs occasionally with commercial power supplies; if a latch stop is invoked whenever it occurs, users may think that there must be a trouble, which is undesirable. Note that phenomena such as a voltage drop and momentary power failure in the AC power supply (commercial power supply) feeding AC power to the switching power supply are hereinafter collectively referred to as “voltage reduction.”
In view of the foregoing, to prevent a latch stop from being caused by a voltage reduction, an IC (integrated circuit) that monitors the voltage level of the AC power may be provided at the primary side so that the control signal for switching is stopped only when the voltage there falls to or below a predetermined threshold level (which needs to be higher than the voltage at which a latch stop occurs) (see, for example, Patent Document 1). With this configuration, it is possible, on one hand, to prevent a latch stop from being caused by a voltage reduction and, on the other hand, to invoke a latch stop when a problem such as short circuiting occurs at the secondary side.
However, providing such an IC at the primary side with a view to detecting voltage reduction in the AC power supply causes an increase in the manufacturing cost of the switching power supply, which is undesirable. Moreover, the IC so provided occupies a large space in the circuit, which is undesirable.                Patent Document 1: JP-A-2005-124252 Publication        Patent Document 2: JP-A-2003-153529 Publication        Patent Document 3: JP-A-2001-119932 Publication        Patent Document 4: JP-A-11-252907 Publication        
It is therefore an object of the present invention to provide a voltage reduction detection circuit for a switching power supply that can detect a voltage reduction in an AC power supply at low cost. It is another object of the present invention to provide a switching power supply system that is provided with such a voltage reduction detection circuit and that can, while avoiding a latch stop invoked by a voltage reduction in an AC power supply, perform a latch stop when short circuiting etc. occurs at the secondary side.